1. Field of the Invention
The present invention relates to an anaerobic digestion process of wastes containing cellulose, specifically an anaerobic digestion process of wastes containing cellulose suitable for liquifactive fermentation by converting efficiently cellulose contained in wastes into volatile fatty acids and for recovering methane after liquefactive fermentation.
2. Description of the Prior Art
Massive quantity of wastes containing cellulose is discharged every year from food processing industries, paper manufacturing industries or lumber industries, including agricultural wastes and livestock wastes, municipal wastes, etc. A part of those wastes are utilized as material for composts, but almost all the rest of the wastes are finally disposed by means of incinerating, landfilling, etc. These means of disposing, however, cause secondary pollution and have problems such as tightened legal restrictions or increasing disposal cost.
Recently, research has been carried out to develop an anaerobic digestion process to dispose of municipal waste in Japan and in the United States.
This anaerobic digestion is a process to convert high-polymeric organic matter such as saccharide, protein or fat into methane and carbon dioxide by the fermenting effect of anaerobic bacteria, consisting mainly of two reactions, i.e., liquefaction in which organic matter such as saccharide, protein, fat, etc., become volatile fatty acids of acetic acid, propionic acid or lactic acid by the action of facultative anaerobic bacteria (which will be hereinafter referred to as "liquefying bacteria") and gasification in which volatile fatty acids produced in the above process are converted into methane by obligatory anaerobic bacteria (which will be referred to as "gasifying bacteria" hereinafter).
When the anaerobic digestion process like this is carried out by a one-step disposal process, a very small amount of bacteria of Cellulomonas or Clostridium which dissimilate cellulose exist in an anaerobic digester, and a part of the cellulose is decomposed, but the speed of decomposition is much slower than that of organic matter which is easy to decompose such as saccharide or protein and is exhausted out of the system before it is changed into methane.
Applicants previously proposed a two-step disposal process in which two reactions of liquefaction and gasification are carried out separately. (U.S. Pat. No. 4,213,857: "Anaerobic Digestion Process", patented on July 22, 1980, M. Ishida et al). By adopting this process, liquefaction and gasification can be carried out separately under the most preferable condition for each of the two reactions.
As a result, time required for disposal can be reduced and this process has a great effect on disposing such wastes as municipal wastes which mainly consists of protein or starches.
However, cellulose is hardly liquefied in a condition favorable for liquefying such decomposable elements as saccharide, protein, fats by the two-step digestion process.
Furthermore, the operation of an anaerobic digestion system for organic matter is generally dependent on past experience. Degrees of digestion have been judged by measuring quantity of generated gas, methane concentration, pH of digested slurry, alkalinity, organic acid concentration, and oxidation-reduction potential of organic matter put into the equipment.
Among these items to be measured, the oxidation-reduction potential has been controlled to be under -500 mV from the empirical viewpoint that digesting efficiency decreases in higher potential than -500 mV, but wastes containing cellulose cannot be sufficiently liquefied in such an operating condition.
On the other hand, liquefying bacteria (facultative anaerobic bacteria) is a group of bacteria being able to grow where oxygen exits, but gasifying bacteria (obligatory anaerobic bacteria) is a group of bacteria which die in the atmosphere containing oxygen (aerobic condition). Accordingly, there is a big difference between the tolerance to oxygen of liquefying bacteria and that of gasifying bacteria.
For the above reason, it is desirable to give priority to the growth of liquefying bacteria by bringing the wastes into contact with oxygen and suppressing the growth of gasifying bacteria. Actually, however, a stable liquefactive effect has not been obtained.